1. Field of the Invention
The present invention relates to a sheet positioning apparatus in a sheet feeding section of a printing press for lateral positioning of sheets of paper.
2. Related Art
In a conventional sheet feeding section, sheets of paper (hereinafter simply referred to sheets) are fed from the top of a stack one by one to a printing section. Specifically, sheets are fed from the stack onto a feeder board (a sheet mounting board) and then conveyed, and prior to being fed to the printing section, the leading edge of each sheet abuts a leading-edge abutment member for fore-aft positioning with respect to the sheet feeding direction, and then sheets are clamped by a pair of upper and lower rollers and then laterally displaced so as to make a lateral edge of each sheet abut a lateral-edge abutment member for lateral positioning. As used throughout the description, the directional term “fore-aft” and “laterally” or “lateral” are used with respect to the sheet feeding direction, in which sheets are fed from a sheet feeding section to a printing section in a printing press, as long as any specific definition is not made.
Sheets to be printed by the printing press have different thicknesses. For a relatively thick sheet, it is possible to securely make a lateral edge of the sheet abut the lateral-edge abutment member. On the contrary, a relatively thin sheet, which is likely to be with poor stiffness, may be forced upward or curled upward at its lateral edge, which results in poor accuracy in lateral positioning. In order to prevent this upward curling of a lateral edge of a relatively thin sheet, a sheet-holding-down member is disposed above the feeder board with a given distance therefrom so as to hold down the sheet from above, which sheet happens to be curled upward at a lateral edge due to abutment with the lateral-edge abutment member. Since a space between the sheet-holding-down member and a sheet on the feeder board is preferably set small enough to achieve efficient holding down of sheets, the sheet-holding-down member as provided is generally designed to be capable of adjusting the height thereof so that where a relatively thin sheet is used, the sheet-holding-down member is moved downward, and where a relatively thick sheet is used, it is moved upward. This type of mechanism is disclosed such as in Japanese Utility Model Application Laid-open No. Hei-2-41937 and Japanese Patent Application Laid-open No. Hei-10-250882.
The above sheet-holding-down member however requires the operator to adjust the height every time sheets are changed to those having a different thickness. This is a troublesome work and also may cause a trouble with sheet feeding when a thick sheet is fed with the sheet-holding-down member held at a lower position.
In consideration of the above problem, it is an object of the present invention to provide a sheet positioning apparatus equipped with a sheet-holding-down member that is capable of preventing upward curling of a lateral edge of a sheet, while reducing the workload of the operator in adjusting the height of the sheet-holding-down member according to the change in thickness of sheet.